In conventional squirrel cage rotors of the above type, the centrifugal forces leads to a large strain of the rotor at high rotational speeds. The maximum attainable speed is limited by a deformation of one or several of the rotor elements or, in extreme cases, by the destruction of the motor. Rotating at high speed, the rotor is storing a considerable amount of rotational energy which may case serious damage in the vicinity of the motor in case of a malfunction.
It is known to use additional elements to protect the short circuit rings against the centrifugal forces acting on them. In document DE-C-42 22 187, the short circuit rings are safe-guarded by additional holding means disposed at the stack of lamination sheets. The holding means are provided at the axial end sheets and engage corresponding recesses provided in the short circuit rings.
Using a solid iron rotor allows high speed rotation due to its high stability, but the maximum rotational thrust, i.e. the force per unit area in the air gap turns out to be unacceptably low. Moreover, the large specific losses make sufficient cooling virtually impossible.